1. Technical Field
This invention relates generally to charging methods for rechargeable battery packs, and more specifically to a charging method that considers both time and temperature in determining how to execute the charging process.
2. Background Art
As anyone who has ever worked on a laptop computer for extended periods of time knows, these devices can get rather warm. Central processors are running at ever higher speeds, and laptop computers tend to generate a lot of heat. This heat creates a problem for the rechargeable battery powering the laptop computer, in that excessive exposure to heat can degrade chemicals and other materials inside the electrochemical cells, thereby reducing their ability to store and deliver electrical energy.
For example, lithium-ion batteries—a popular choice for today's portable computers—include a liquid electrolyte that facilitates the flow of ions from the positive terminal to the negative terminal, and vice versa. This electrolyte, in addition to other battery chemicals, can react with active materials in the battery when exposed to high temperatures. Additionally, other materials and components in a battery, including electrode materials and separator materials, may degrade in the presence of high temperatures. The degradation causes the total amount of energy that can be stored in the battery's cells to become diminished. Simply put, a battery that originally provided four hours of run time may only provide two hours of run time after excessive exposure to heat.
One particularly harsh environment for a portable computer occurs when it is attached to a docking station. For instance, many people no longer use desktop computers. Instead, they opt for only a laptop computer. When they are at the office or at home, they connect the laptop computer to a docking station so as to use the laptop as if it were a desktop. The docking station generally includes peripherals like a monitor, keyboard, power supply and printer.
When the laptop computer is coupled to a docking station, the attached power supply charges the battery (if needed) and then continues to power the computer. Due to the small size of modern laptop computers, the fully-charged battery is often in close proximity with the hot central processor and other accompanying components. Consequently, the continued use of the computer—and the corresponding heat generated by the computer's electronics—causes the fully charged battery to become quite warm, thereby compromising reliability of the battery.
There is thus a need for an improved method of managing the state of charge of a rechargeable battery so as to prevent thermal deterioration due to excessive exposure to heat.